It should be possible, I did play around with it briefly. However we found it easier to get it up as a homing function.
G28.2Z (did you update your firmware to the CNCbin?)
The homing retract from homing should set the head the correct distance from the material.
if its not in perfect focus try adjusting this value.
gamma_homing_retract_mm 2.5 # "
You can then of course move the bed down by 10mm for example and set 0 this will then give you a wider out of focus beam.
If you want to set 0 of just the Z type G92 Z0